A display sheet and display system is disclosed in a copending patent application, assigned to the same assignee as this application, U.S. Ser. No. 07/433,311 filed Nov. 8, 1989 and now abandoned, entitled "Paper-Like Computer Output Display and Scanning System Therefor" which is fully incorporated herein by reference. Also incorporated herein by reference are U.S. Pat. Nos. 4,126,854 and 4,143,103 and an article entitled "The Gyricon--A Twisting Ball Display", published in the Proceedings of the S.I.D., Vol. 18/3&4, Third and Fourth Quarters 1977.
The display device, in sheet form, as described in my copending application comprises a thin transparent sheet having many of the attributes of paper documents. It looks like paper, has ambient light valve behavior like paper (i.e. the brighter the ambient light, the more easily it may be seen), flexible like paper, can be carried around like paper, can be written on like paper, can be copied like paper, and has nearly the archival memory paper. It is also possible to form the display device as a rigid structure incorporating an array of addressing electrodes. In both embodiments, the salient features are an elastomeric host layer a few mils thick which heavily loaded with balls tens of microns in diameter. Each bichromal bail has hemispheres of contrasting colors, such as a white half and a black half, and is contained in its own spherical cavity filled with a dielectric liquid. Upon application of an electrical field between electrodes located on opposite surfaces of the host layer, the balls will rotate depending on the polarity the field, presenting one or the other hemisphere to an observer.
In the above-identified article, there is disclosed a method for fabricating bichromal balls. First, monochromatic glass balls are formed, heavily loaded with titanium dioxide so as to appear white. These are deposited in a monolayer upon a substrate. Then the balls are coated from one direction in a vacuum evaporation chamber with a dense layer of nonconductive black material which coats one hemisphere.
As illustrated in FIG. 1a bichromal balls 10 are loaded in liquid filled cavities 12 in a host matrix 14. Both the liquid 16 surrounding the balls and the balls themselves are dielectric. Therefore, although the balls are macroscopically electrically neutral, on a microscopic scale they have an electrical double layer comprising two layers of charges of opposite sign (as shown). One charge layer is localized at the surface of the ball and the other charge layer is in the nature of a space charge extending outward from the surface of the ball into the dielectric liquid. The measurable aspect of the electrical double layer, known as the zeta potential, is the net surface and volume charge that lies within a shear surface associated with the motion the ball through the liquid. For a given liquid, the zeta potential is a function only of the ball surface material. Thus, the material properties which give rise to differences associated with the color or reflectivity of each hemisphere 18 and 20 give rise to different characteristic zeta potentials with respect to the dielectric liquid 16 in the cavity 12. It is the difference in zeta potential between the hemispheres of the ball which causes the ball to act like a dipole in the presence of an electrical field, as illustrated in FIG. 1b. The ball 10 will rotate, until its dipole moment lines up with the direction of the electrical field established between opposed electrodes 22 and 24.
In addition to the dipole charge distribution found on the bichromal ball the presence of an electrical field. there is also a monopole charge which the net electrical charge. It is quite unlikely that the two hemispheres and 20 having zeta potentials of opposite polarity will have the same magnitude. However, if that is the case, a monopole charge will not be established. As a result of the monopole charge, the ball 10 is caused to translate in the direction of the electrical field and will rest and be retained against the cavity wall, as illustrated in FIG. 2. In order for the ball rotate easily in the liquid within the cavity, due to the dipole charge, it must move from contact with the cavity wall. When at rest against the cavity wall, friction and other forces will prevent it from rotating until it has been moved away once again, due to the monopole charge. It is this feature which enables long term image retention in this display device.
In my copending patent application, identified above, of which this application is a continuation-in-part, there is disclosed the flowing together of two side-by-side streams of differently colored hardenable liquids into the center of a laminarly flowing host liquid. The forward end of the thus introduced bichromal stream becomes unstable and breaks up into droplets which form into spherical balls as they are moved by the host liquid Further transport of the balls by the host liquid moves them past a curing station and a separating station.
It is an object of the present invention to generalize the invention of the copending parent application to include expelling the side-by-side multichromal stream, into a fluid as one or more free jets, i.e. one not constrained by any solid surface.